1. Field of Invention
The present invention relates to an organic electroluminescence element which is excellent in the output efficiency of light generated in the organic luminous layer.
2. Description of Related Art
Organic electroluminescence displays which are equipped with organic electroluminescence elements (elements having an organic luminous layer between the cathode and the anode) corresponding to the respective pixels have high brightness with spontaneous light. The organic electroluminescence displays can be driven with direct current at low voltage, have a high speed response, and have light generated by a solid organic film. Thus, the organic electroluminescence displays are excellent in display performance and enabling thickness reduction, weight reduction, and electricity consumption reduction. The organic electroluminescence displays are therefore expected to replace liquid crystal displays in the future.
In an organic electroluminescence display, a large number of pixels, formed of organic electroluminescence elements, are placed in a matrix, that is formed of rows and columns intersecting at right angles. The active matrix method and the passive matrix method are methods of driving organic electroluminescence displays.
In the passive matrix method, patterning is performed by enabling one of the two electrodes, which sandwich an organic luminous layer, correspond to the row and the other to the columns. A pixel formed of an organic electroluminescence element is formed in the position where the two electrodes overlap. Also, because the row electrode and the column electrode correspond to the scanning line and the data line, and the ON state is made by selecting one row electrode and one column electrode, only the pixel in the position where both electrodes are simultaneously in the ON state emits light.
On the other hand, in the active matrix method, one electrode (the pixel electrode) and the organic luminous layer are formed in a matrix shape. The other electrode is formed over the entire surface of the display as the common electrode, and each pixel is equipped with a driving transistor and a capacitor. Therefore, an active matrix type organic electroluminescence display enables higher definition at high brightness, and is therefore able to deal with increases in gray scales and display size.
An explanation is provided of an example of active matrix type organic electroluminescence displays discussed above with reference to FIGS. 9-11. FIG. 9 is a partial plane view, showing one pixel and its driving element, etc., that surrounds the pixel. FIG. 10 shows a circuit for driving one pixel of the display. FIG. 11 is a cross-sectional view taken along plane A—A of FIG. 9.
As shown in FIGS. 9 and 10, in this active matrix type organic electroluminescence display, each pixel, formed of an organic electroluminescence element E, is equipped with a switching transistor 13, a capacitor 14, and a driving transistor 15. These elements are connected to the driving circuit via a scanning line 10, a signal line 11, and a common line 12. In this active matrix type organic electroluminescence display, a pixel is selected by a switching transistor 13. An organic electroluminescence element E, which is the pixel, which permits the emission of light at a preset brightness by a driving transistor 15.
As shown in FIG. 11, on a glass substrate 1 of this display, after each driving element, including a signal line 11 and the driving transistor 15, are formed, an insulating layer 16 is formed. In this insulating layer 16, a contact hole 16a is formed on the position of the source/drain electrode 15a of the driving transistor 15. Also, a bank 17 is formed on the insulating layer 16. The bank 17 divides the substrate surface into pixels.
In the pixel area divided by this bank 17, an anode layer (light-transmissive electrode layer) 3 and an organic luminous layer 4 are formed. Further, a cathode layer (light-reflective electrode layer) 5 is formed on the entire substrate above the bank 17 and organic luminous layer 4. When forming the anode layer 3, the contact hole 16a is filled with the component material (conductor) of the anode layer 3, connecting the source/drain electrode 15a and the anode layer 3. In FIG. 9, a conductor (connecting plug) filled in this contact hole 16a is indicated by the reference number 18.
Enhancing luminous efficiency of the organic electroluminescence element is an effective technique to reduce consumption of electricity by an organic electroluminescence display, irrespective of the difference in driving methods. Efficiency is enhanced by enhancing the raw materials and their combination in each layer of the hall transport layer and/or the electron transport layer between the organic luminous layer and electrode layer.
Also, as shown in FIG. 12, light generated in the organic luminous layer 4 radiates in all directions. Then, the light irradiated straight toward the side of the glass substrate 1A, and a part of the light reflected by the interface between the light-reflective electrode layer (cathode layer) 5 and the organic luminous layer 4, emerge toward the side of the glass substrate 1A. As shown in FIG. 12, in a cumulate body where the whole cumulate surface is flat, light H irradiated in parallel with the cumulate surface of the cumulate body S travels toward the end surface of the organic luminous layer 4 (the surface in contact with the inner wall of the bank 17 in FIG. 11) and does not emerge toward the side of the glass substrate 1A.
As a result, outgoing efficiency of light generated in the organic luminous layer 4 (ratio of light quantity emerging to the side of the glass substrate 1A to the total emission quantity in the organic luminous layer 4) is approximated as being about 20%, for example. Therefore, increasing outgoing efficiency of light generated in the organic luminous layer 4 becomes important for reducing consumption of electricity by the organic electroluminescence display.
Japanese Patent Publication Hei 11-214163 discloses that outgoing efficiency of light generated in an organic luminous layer is increased by reflecting, in the direction perpendicular to the substrate surface the light outgoing in a horizontal direction relative to the substrate surface, by installing many holes in one electrode layer and installing slopes on the other electrode utilizing these holes.